This invention relates to a method of, and apparatus for, estimating frequency characteristics of a power system. More particularly, the present invention relates to a technology, and its application, of receiving on-line information of generators and governors and appropriately estimating frequency characteristics of a power system on the basis of the on-line information.
A supply-and-demand imbalance and a frequency deviation have a close relationship in the operation of a power system. This relationship of the frequency deviation with the supply-and-demand imbalance is generally referred to as xe2x80x9cfrequency characteristics of a systemxe2x80x9d. The frequency characteristics must be grasped for planning and operating a power system.
For example, the frequency characteristics are utilized in frequency stabilization apparatuses. To correct the frequency to an appropriate range to cope with an abrupt change of the frequency for some reason, the supply-and-demand imbalance must be solved by quick generator shedding and load shedding to suitable levels. To calculate a supply-and-demand imbalance resolution quantity necessary for absorbing the frequency fluctuation at this time, it is essential to grasp the frequency characteristics of the power system. Therefore, an estimated value of the frequency characteristics calculated empirically is set to the power system as a control object in the conventional frequency stabilization apparatuses.
The frequency characteristics of the power system must be grasped not only for the frequency stabilization apparatus but also for monitoring the frequency in the restoring operation of the accident of the power system, for example. In this case, too, the conventional apparatus makes use of the set value determined empirically.
JP-B-7-108063, for example, discloses an example of the frequency stabilization apparatus of a separation system. The prior art reference simulates an assumed separation system to a one generator-to-one load model and sets empirical frequency characteristic coefficients to the generators and the loads, respectively.
However, the prior art technology has the limits in improving control accuracy because it does not take into consideration the change of the frequency characteristic coefficient with the change of the conditions of the generators and the loads resulting from the change of the operation condition.
Particularly because the frequency characteristics of the generators are the main factor of the frequency characteristics of the overall system, it has been desired to dynamically estimate the frequency characteristics of the power system in match with the generator-group configuration in the case of the system separation operation that assumes great changes in the generator-group configuration and in the operation condition.
The frequency stabilization apparatus described in JP-A-4-140022 enables the frequency characteristics of the power system to be adapted to the supply-and-demand imbalance. However, because this prior art does not take the change of the operation condition of each generator as the main factor of the frequency characteristics into consideration on one hand and because it depends on the actual measurement of the frequency itself of the power system, on the other hand, this frequency stabilization apparatus involves the problem that it cannot easily adapt itself to any abrupt change of the supply-and-demand condition of the generators, such as generator shedding.
It is an object of the present invention to provide a method of, and an apparatus for, estimating frequency characteristics of a power system in accordance with the operation condition of the power system.
The frequency characteristics of the power system depend on the frequency characteristics of the generators and on the frequency characteristics of the loads, but the frequency characteristics of the generators are predominant because drooping control is applied to the generators. Incidentally, it is practically difficult to estimate the frequency characteristics of the overall load in match with the supply-and-demand condition because the frequency characteristics of the individual loads must be estimated by measuring the operation conditions of all the loads. Therefore, it has been inevitable in the past to conduct the estimation on the basis of a statistical processing. Nonetheless, the information necessary for estimating the frequency characteristics of the generators such as the information about the generators connected in parallel in the system, the information about the construction of the governor provided to each generator and the information on the operation condition, for example, can be easily measured on an on-line basis.
According to one aspect of the present invention, a method of estimating frequency characteristics of a power system for accomplishing the object described above comprises the steps of defining a sensitivity coefficient of power control of generators with respect to a frequency deviation, such as a proportional constant (1/delta), as the frequency characteristics of the generators; grouping power controllable generators of the power system into a generator group and determining the frequency characteristics of the generator group; multiplying the frequency characteristic coefficient of the individual generator by the proportion of its rated output to the rated outputs of all the generators, adding the products so obtained for all the generators and determining the sum obtained thereby as the overall frequency characteristic coefficient; and performing the above-defined operations for power controllable generator groups gathered in accordance with different supply-and-demand conditions and providing estimated frequency characteristics of the power system with respect to the overall supply-and-demand condition of the generator group, that is, the supply-and-demand imbalance quantity.
In other words, because the sensitivity characteristics of power control of the generators with respect to the frequency deviation correspond basically to the frequency characteristics of the generators, the sensitivity coefficient of power control of each generator is defined as the frequency characteristic coefficient of the generator. Simply speaking, the frequency characteristic coefficient of a generator group comprising a plurality of generators can be determined by adding the frequency characteristic coefficients of a plurality of generators belonging to the power system. However, the power control quantity becomes different if the generators have any difference in their rated outputs and in such a case, their contribution ratio to the frequency characteristics becomes different, too. Therefore, weighting is applied to the frequency characteristic coefficient of the individual generator in accordance with its rated output and the frequency characteristic coefficient of the generator group comprising a plurality of generators is then determined. In this way, the frequency characteristic coefficient of the generator group can be determined appropriately.
In this case, the frequency characteristic coefficient of the generator group can be determined by, for example, multiplying the frequency characteristic coefficient of the individual generator by its rated output, summing the product so obtained for all the power-controllable generators and dividing the sum by the total output quantity of the generator group.
The frequency characteristic coefficient of the overall power system is then determined by adding the frequency characteristic coefficient of the overall load to the frequency characteristic coefficient of the generator group estimated in the manner described above. This frequency characteristic coefficient of the overall power system is a coefficient that associates the supply-and-demand imbalance with the frequency deviation and can be expressed by a segmented function.
In the description given above, the frequency characteristic coefficient of the generator can be determined on the basis of the droop (xcex4) of the governor so provided as to correspond to each generator. The frequency characteristic coefficient is preferably determined for those generators which have a margin of output control inclusive of reserve power. Preferably, further, the operation conditions of the generators are divided into (a) the first state where all the generators have the margin of output control; (b) the second state where a part of the generators is limited in power control; (c) the third state where a part of the generators reaches the limit of power control while the remaining generators still have the margin of power control; and (d) the fourth state where all the generators reach the limit of power control inclusive of reserve power, the supply-and-demand imbalance is so divided into four regions as to correspond to these four states, respectively, and the frequency characteristic coefficient of the generator group is determined for each of these regions. According to this method, the frequency characteristics of the power system can be estimated appropriately in accordance with the practical operation condition by taking the load limit of the generators into consideration.
More specifically, data as to paralleled generators or separated generators and the data on the operation condition are collected on the basis of the generator outputs as the on-line observation values, and the operation method of each generator is grasped. Here, the term xe2x80x9coperation methodxe2x80x9d represents a load reference value corresponding to the generation output required by a control station that monitors the supply-and-demand condition of the overall system, the load limit value of the generators, and so forth. The term xe2x80x9cload limit valuexe2x80x9d of the generator represents the range of the limiter of the governor grasped from the data base of the rated generator setting data in which the governor can control or the range in which its operation is limited. The range in which power control of each generator can be made is grasped from these generator output and operation method, and the supply-and-demand imbalance region is divided by gathering such ranges into the zones in which the power controllable generators belong to the same group. The frequency characteristics of each generator group are estimated by taking the power controllable generators into specific account for each of the divided supply-and-demand imbalance regions and furthermore, the frequency characteristics of the loads estimated by known separate means are added to obtain the frequency characteristics of the overall system, and the frequency characteristics of the power system inside a specific supply-and-demand imbalance region are estimated.
Incidentally, it is known that the load reference values of the generators and the load limit values change depending on the season and on the time of each day, and the load reference value and the load limit value are assumed for each season and for each time on the basis of the known data and are stored in memory means. The frequency characteristics of the power system can be determined by adding the season and the time as the specific factors.
As the frequency characteristics of all the generators, that is, the frequency characteristics on the supply side, are estimated by the calculation which is suitable for the construction of the generators connected to the system and their operation condition which is made in time as described above, the frequency characteristics of the system can be grasped more accurately. Because the present invention can immediately cope with the change of the generator configuration without waiting for the measurement of the frequency, the present invention can improve accuracy of control in the frequency stabilization apparatus and in various other operations, and can execute planning or a restoring operation with high reliability when applied to planning of the power system and simulation of its restoring operation.
These and other objects, features and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments taken in conjunction with the accompanying drawings.